In the phases of matter normally considered by the lay person there are a series of material characteristics typified, in a cooling sequence, by a gas phase, a liquid phase and a solid phase. For materials such as simple organic liquids (e.g. methanol, hexane etceteras) this is normally sufficient. However, as the molecular constituents become longer, more rigid or more complex, these phase sequences are liable to become more complex. In the very large molecules, comprising many tens or many hundreds of thousands of units, the most familiar behaviours are those of polymers which may show no gas phase (because the molecules decompose below any temperature where the systems can “evaporate”) but may have several properties upon cooling through a solid-like state, for example a rubber, a glass and a crystal, in sequence. For slightly smaller molecules the situation may be even more complex; herein we may find a gas phase, a first liquid phase (which is typically isotropic), a second liquid phase (which is light scattering) and possibly several, distinct, further “liquid-like” states prior to freezing, as a solid, which may be crystalline or not. These more complex liquid states are very often indicative of liquid crystalline behaviour. As the phrase (or designation) suggests, these liquids have molecules which have a propensity to self order without freezing and thus gain crystalline attributes even though they still flow and may fill a container.
The phases of liquid crystals are many and complex but may be readily (albeit broadly and non-comprehensively) described as a generalised sequence of states that such a molecular fluid may pass through on the way from being an isotropic liquid until it freezes as a solid. In general such molecules will be typified by strong anisotropy. The form that this anisotropy takes may be complex, but for the present purposes, cases can be considered where the molecule is typified by a high aspect ratio (much longer than wide, thus “rod” or “lathe” like), and may have dipole character, and anisotropic polarisability. In these cases the average direction of molecular orientation is referred to as the “director”. Very often such properties of anisotropy are well aligned with each other, but significant cases exist where the forms of anisotropy of a molecule do not have the same principle axes within a Cartesian coordinate system. Such molecules are of profound significance in a very large class of materials of biological and physical scientific interest.
Nematic liquid crystals typify the commonest liquid crystalline materials and are commonly used in liquid crystal flat screen devices and flat-panel displays. They are typically fairly short (bi-phenyl) aromatic cores with a charge transfer character (in the extreme), a strong electron donor and acceptor group serving to enhance polarisability, and with modest head or tail extensions which enhance their mesogenic character (molecular aspect ratio, internal heterogeneous character). Such nematic materials typically display a uniaxial order whereby they have an anisotropy displayed along a certain axis and the plane normal to this axis has little or no anisotropy. However, the nematics are still relatively fluid and if they have strong dipolar and polarisable character they may be aligned with an electric field (or magnetic field) dielectric re-orientation axis. This is the principle behind many of their most valued applications. They may also be ordered by alignment agents, physical flow and other mechanical processes, and, in various applications, these processes are very often used to set a pre-determined initial condition or a state to which they will return after perturbation. Generally, in the absence of strong alignment agents, or a situation wherein the nematic is constrained in an anisotropic field, then upon removal of an imposed field the nematic liquid crystals will relax to poly-domains that are locally anisotropic, and thus appear optically “light scattering”.
Extending the length of nematic mesogens, or other structural changes, very often causes them to show further phases upon cooling below the nematic phase, and before solidification, and at lower temperatures the typical character may be of a “layered fluid”. X-ray and other studies show that a weak density wave, characteristic of well defined layer spacing, develops, and the materials are distinct from the nematic phases both via microscopy and in visco-elastic and other character. These layered liquid crystals are called “smectic” liquid crystals. Herein we will only consider the materials normally referred to as “Smectic type A”, or just “smectic A”, abbreviated to “SmA”, liquid crystals. For example the proto-typical “5CB” (4′-pentyl-4-biphenylcarbonitrile), “5OCB” (is the ether linked pentyl, 4′-(pentyloxy)-4-biphenylcarbonitrile), is nematic, the “8CB” (4′-octyl-4-biphenylcarbonitrile) and “8OCB” (4′-(octyloxy)-4-biphenylcarbonitrile), each exhibit a SmA phase beneath the higher temperature nematic phase.
The following convention is used within this specification for the abbreviation “mCB” and “mOCB” where m stands for an integer and refers to the number of carbon atoms in the alkyl or alkoxyl chain in 4-cyano-4′-n-alkylbiphenyl and 4-cyano-4′-n-alkoxybiphenyl, respectively; for example:
8CB=4-cyano-4′-octylbiphenyl; and
8OCB=4-cyano-4′-octyloxybiphenyl
Other abbreviations used in the specification are set out in the Tables at the end of the specification.
The molecules forming SmA phases have similar properties to those forming nematic phases. They are rod-like and usually have a positive dielectric anisotropy. The introduction of a strong transverse dipole in order to induce a negative dielectric anisotropy tends to destabilise the SmA phase and may lead to increased chemical instability.
One particular character of smectic liquid crystals is a marked hysteresis in their switching to the extent that dielectric re-orientation (or other disturbances of the smectic structure) do not relax when the electric field is removed e.g. sec Crossland et al ref. P4 and ref 6), i.e. unlike most nematic liquid crystal structures, dielectrically re-oriented SmA liquid crystals remain in the driven state until further forces are applied. This is explained via reference to the nature of the processes which are used to drive such liquid crystals. A brief description of the nature of the order in such liquid crystals is provided here to clarify the discussion below. By definition, SmA liquid crystal compositions form a layered structure. In a body of SmA material, the layers of the liquid crystals in the different regions of the body may be more ordered with respect to each other or less ordered. (i.e. more disordered) As used herein the terms “ordered” and “disordered” refer to the alignment of the layers within a body of SmA liquid crystal composition. In an extreme case of a disordered state, the composition is broken up into fragments (or domains) and the orientation of the layers within each fragment is not influenced by its surroundings, including the orientation of the layers in neighboring fragments. However, that is an ideal situation and in practice, the liquid crystal system will have constraints upon it, such as the juxtaposition of walls containing the composition, especially field electrodes, and these will cause some deviation from a truly random (stochastic) alignment of the layers in the fragments of the layered phase. Similarly there will be some residual alignment of the layer orientation after the composition has been disturbed: such a distribution is often parameterised by reference to mathematical definitions of “order parameter” (e.g. see reference 15). In other words, even in an extreme case of disorder, the orientation of the layers in the different fragments will not be completely random; such a state of affairs is often referred to as “pseudo-random”.
As a field is applied to the composition, the nature of these layer distributions will tend to change and the layers in the various fragments will statistically become more aligned with the electrodes and with each other, i.e. more ordered, and this ordering will asymptotically approach to a mono-domain in which all the layers in the composition are perfectly ordered, i.e. aligned with each other, and so the fragments become a single domain. However, such a perfectly aligned system is generally also an idealized state.
There will be intermediate states of alignment at which there would be a definable order parameter intermediate between the starting (disordered) condition and the (ordered) end condition (in the case of going from disorder to order this parameter would be increasing). The end condition itself would approach a defined value of the order parameter, which is often expressed as a normalised value between 0.0 (no order) and 1.0 (fully mutually aligned). In the latter case we would have approached a perfect mono-domain of completely correlated alignment of the layered states.
A thin glass cell may be formed by taking planar sheets, generally of glass (similar to small microscope slides), and applying to these a transparent conducting layer, typically made of indium tin oxide. These two sheets may be formed into a thin cell for example separated by beads of uniform diameter (typically, say, 5-15 micrometers, dependent on desired cell thickness). This cell is normally edge sealed with glue allowing apertures for filling (only one for small cells vacuum filled, but two or more in flow or pumped filling systems) with the liquid crystal. Such simple glass cells are very often used for liquid crystal characterisation and are similar in form to the much larger glass panels used in display devices (for nematic liquid crystals, these generally have much thinner cell-gaps). Using such a cell a SmA liquid crystal layer may be formed by filling the cell (typically at an elevated temperature above the isotropic transition for the material). In the SmA devices discussed here, no alignment layers need be used in such materials, in strong contrast to nematic display-type devices where uniform alignment of the cell is a requisite of their operation. On filling and thermally cycling such a thin SmA cell from room temperature to above the isotropic transition and back again, the liquid crystal will exhibit textures that are typical for the phases. Whilst the nematic, with no surface alignment, may appear in the well-known Schlieren texture where line defects or ‘threads’ scatter light, in the SmA a ‘focal conic’ texture is formed as a consequence of the layered structure of the SmA material. There is a sharp spatial variation in the refractive index which can result in light scattering (photo-micro-graphs of the liquid crystal textures are shown in FIGS. 2 to 5). The appearance of these textures is a consequence of the anisotropy of the refractive index, which is highest when light is travelling orthogonal to the more polarisable axis of the average molecular direction. The variation in refractive index causes light scattering. When viewed (under a microscope) between crossed polarisers, contrast can also be observed between regions of different molecular orientations.
In such cells SmA materials may have their electro-optic responses measured. The application of wires to contact the conducting glass coating allows the field across the liquid crystal layer to be established and modulated.
To electrically address a SmA liquid crystal an alternating (AC) field is normally applied. For non-doped materials with no ionic contamination or additives, the dielectric anisotropy of the LC will cause the re-arrangement, of the initially randomly aligned poly-domains, to align the mesogen with the field direction (normal to the glass surface). Under such a condition, the cell (viewed in transmission or normal to its surface), will appear clear. In this condition the average orientation of the anisotropic molecules is normal to the glass layer. We can say the SmA layer is now in a mono-domain, that is oriented with layers parallel to the glass plates. For most SmA materials this situation is only reversible by re-heating the cell to destroy the SmA alignment.
Most SmA materials have a positive dielectric anisotropy, i.e. the average direction of the long axis of the molecules will align with an electric field. A film of smectic A liquid crystal aligned in this manner between glass plates has the average orientation of the long molecular axis (called the ‘director’) aligned orthogonal to the glass plates. This orientation is referred to as ‘homeotropic alignment’.
This dielectric re-orientation of smectic A liquid crystals, with positive dielectric anisotropy, cannot alone form the basis of a practical electro-optic phenomenon for use in display devices because it can only be reversed by heating and subsequent cooling. A smectic crystal film between glass plates, as described above, that has been uniformly dielectrically re-oriented, appears clear and transparent or as an oriented waveplate when viewed in polarised light (i.e. if the cell is viewed between sheets of linear polarising film). Two methods of generating optical contrast relative to this state have been demonstrated: Contrast can be generated by using another electric field to re-orient the waveplate. The change is visible if this is viewed between sheets of linear polarising film. Alternatively light scattering can be electrically induced in the layer by disrupting the mono-domain. This is visible without polarised light.
It is possible to employ liquid crystals with a negative dielectric anisotropy at low frequencies and a positive dielectric anisotropy at higher frequencies (so-called two frequency materials) as described by Crossland et al 1978 (refs 6 and P4) and in this instance, it is possible to reversibly switch such a waveplate using dielectric re-orientation. However the molecular structures required, inducing negative dielectric anisotropy at low frequency, usually conflict with the requirements for stable SmA phases and reduces significantly the value of the positive dielectric anisotropy at higher frequencies (so both re-orientations require relatively high drive voltages and are relatively slow).
Here we are concerned with the better prospect of reversibly switching between a homeotropically aligned clear transparent state and a disordered light scattering state created by smectic dynamic scattering (SDS):
If a suitable ionic dopant is dissolved in the smectic A liquid crystal host, then under the influence of DC or low frequency (e.g. <500 Hz) electric fields, two orthogonal forces attempt to orient the smectic A director. Dielectric re-orientation as described above attempts to align the smectic A director (indicating the average direction of the long molecular axis) in the field direction. Simultaneously, the movement of ions through the smectic A electrolyte attempts to align the smectic A director in the direction in which ions find it easier to travel. In smectic A materials this is within the layers i.e. orthogonal to the field direction (i.e. the materials have a positive dielectric anisotropy and a negative conductivity anisotropy). The two competing forces give rise to an electro-hydrodynamic instability in the liquid crystal fluid that is referred to as ‘dynamic scattering’. (It is analogous to the similar process in nematic liquid crystals where the conductivity anisotropy is positive, so it only occurs in nematic liquid crystals with a negative dielectric anisotropy.) In smectic A materials the dynamic scattering state strongly scatters light and (in contrast to the similar state in nematic materials) the disruption of the smectic A structure that it produces remains after the electrical pulse causing it has terminated. The reversibility between the clear, uniformly oriented, state and the ion-transit induced, poly-domain, scattering state, depends upon the different frequency domains in which these processes operate. Dynamic scattering requires the field driven passage of ions through the liquid crystal fluid. It therefore occurs only with DC or low frequency AC drive. Higher frequencies cause dielectric re-orientation (the ions cannot “move” at these frequencies) thus re-establishing a uniform orientation of the molecules.
Thus the combination of dielectric re-orientation (into a clear transparent state) and dynamic scattering (into a strongly light scattering state) in a suitably doped SmA phase (possessing a positive dielectric anisotropy and a negative conductivity anisotropy) can form the basis of an electrically addressed display and is used in the present invention. High frequencies (variable, typically >1000 Hz) drive the SmA layer into an optically clear state and low frequencies (variable, typically <500 Hz) drive it into the light scattering state. A key feature of such a display is that both these optical states are set up using short electrical pulses, and both persist indefinitely, or until they are re-addressed electrically. This is not true of the related phenomena in nematic liquid crystals. It is this property of electro-optic bistability (or more accurately multi-stability) that allows SmA dynamic scattering displays to be matrix addressed without pixel circuitry and which results in their extremely low power consumption in page-oriented displays or in smart windows.
The phenomenon of dynamic scattering in SmA liquid crystals was predicted by Geurst and Goosens in 1972 (ref 8). It was first observed and identified by Crossland et al 1976 (ref P1) who proposed displays based on this phenomena and dielectric re-orientation and described their structure and electrical addressing (refs. P1, P2, P3, 1, 2 and 3). Subsequently highly multiplexed passive matrix displays were demonstrated with good viewing characteristics based on efficient switching between clear and scattering states (refs 4). Such displays are generally viewed against a black background and could be illuminated (e.g. with a front plastic light guide) or used without illumination as reflective displays. They were also used as efficient projection displays because the clear areas are highly transparent (no polarising films) and the scattering textures efficiently scatter light out of the aperture of projection lenses.
A second method of generating contrast using the electro-optic effects was also disclosed in P1 (Crossland et al 1976): if a suitable dichroic dye is dissolved in the SmA then the dye orientation is randomised in the scattering state, which therefore appears coloured. The clear state however orients the dye orthogonal to the liquid crystal layer so its absorption band is not apparent. This ‘guest-host’ effect (where the dye is the guest in the SmA host) switches between the dye colour and white when viewed against a white background. Displays were fabricated using dyes of various colours (including mixtures of dichroic dyes to give black) and devices employing, for example, anthraquinone based dyes exhibited good contrast and photochemical stability. Such dye can also be used in the present invention.
This invention relates to displays as described in which a disordered state is produced by the process of SmA dynamic scattering and a clear, uniform state is induced by dielectric re-orientation. Here they are referred to as smectic A dynamic scattering (SDS) displays. These two states are equally stable allowing arrays of pixels of any size to be addressed line-at-a-time without the use of pixel circuitry. Such line-at-a-time display drivers are well known.
Their nature has not heretofore been attractive for main-stream video display development where nematic liquid crystals have largely been favoured. However, with emergent requirements for electronic display systems of superior energy efficiency the SmA materials offer several significant intrinsic advantages. In particular SmA materials are very attractive for information displays where video performance is not requisite and high energy efficiency, and quite possibly unlit operation, is desired (reflective display systems).
A typical example is provided by the consideration of metropolitan information systems (e.g. displays of road-traffic information, public transport timetables, visitor information etceteras). Such will need to operate in a quasi continuous up-date mode, with some sites requiring full exposure to sunlight, others being sited where frequent maintenance is difficult. Such applications will thus require refresh rates that are reasonable and provide a readable experience (for comparison, consider the experience of reading and turning a page of a book or magazine). Similarly with continuously refreshed, paged, data, the expectation for acceptable lifetime must suggest that the screen can be refreshed many times, say, for a service life of 3 to 5 years (if we assume pages will be refreshed every 10 seconds then this would imply that the display must operate between 10 and 15 Million refresh cycles). Naturally this operation scenario is not the only consideration, but it does provide a useful guideline for the fabrication of practical devices.
The use of SmA SDS in reflective, and front and/or back-lit, display systems goes back to the 1970s and 1980s when early trials of SmA materials in a scattering display mode were evaluated for point-of-sale display, information systems, electronic books and electronic displays for overhead projectors (see Crossland et al ref 4). The choice between using dyed or un-dyed systems has historically been dependent upon application specifications.
For simple monochrome applications the use of dyed or un-dyed systems is viable, in the latter case a printed back-drop may be used to present a colour when the material is cleared. In both cases the scattering texture is critical to the visual quality of the display. For the dyed cases the contrast is between the (normally) white of the back-drop and the achieved extinction of the light from the dyed scattering texture. In the un-dyed system the contrast perceived is through the achieved background scattering ‘brightness’ (of the native scattering texture) and the contrast between that and the revealed back-drop. In both of these cases the texture which develops in the scattering state and the ability to clear that state back to ‘transparency’ are critical application parameters. Light scattering depends upon both the refractive index anisotropy of the material and the scale of the micro-structure developed in the scattering state. In liquid crystals these are related through several equations which tie dielectric anisotropy and other parameters together with field driven character.
The background on SmA liquid crystals as a phase of matter is widely covered in the liquid crystal literature (e.g. in the books ref 9).
The possibility of dynamic scattering on SmA phases was postulated by Geurst and Goosens 1972 (ref 8). It was first observed by Crossland, Needham and Morrissy (1976 ref P1 and subsequent references).
From the fundamental studies and theoretical development we may expect, simplistically, that field induced structural inhomogeneity will arise with scales set by,w∝K1/2/E,  Equation 1Where, w is a characteristic length scale (domain size), K is the effective elastic modulus and E is the applied field.
The time over which such inhomogeneity may emerge is related to,τ∝η/E,  Equation 2Where, τ is the time, η is the effective viscosity and E is field.
Dynamic scattering relies on the competing forces of ‘flow alignment’ due to the passage of ions and dielectric re-orientation attempting to align the liquid crystal director in orthogonal directions. The voltage required to cause scattering scales with a relationship derived by Geurst and Goosens. In their paper they relate the threshold voltage V, to the ratio of the product of the effective elastic tensor component, K, (for smectic A liquid crystals this is the splay component, K11) and the cell thickness; divided by the product of the dielectric modulus (reduced by a factor related to the conductivity anisotropy) multiplied by the characteristic length, λ, of the smectic layer, thus,
                                          V            scatter            2                    =                                    2              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              K                11                            ⁢              d                                                      ɛ                0                            ⁢                                                ɛ                  parallel                                ⁡                                  (                                      1                    -                                                                  σ                        parallell                                            /                                              σ                        normal                                                                              )                                            ⁢              λ                                      ,                            Equation        ⁢                                  ⁢        3            
The companion to this relation is that which drives (at higher frequency) the re-alignment of the molecular dipoles to re-orient the molecular axes normal to the glass surfaces, parallel to the field, thus,
                                          V            clear            2                    =                                    2              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              K                11                            ⁢              d                                                                        ɛ                  0                                ⁡                                  (                                                            ɛ                      parallel                                        -                                          ɛ                      perpendicular                                                        )                                            ⁢              λ                                      ,                            Equation        ⁢                                  ⁢        4            
From this scenario we might note that the factors critical to optimizing operational performance are the dielectric (also refractive index) and conductivity anisotropy. These observations are certainly true and have been validated by much of the literature, however, the factors K and λ also have critical operational implications.
                              λ          =                                    (                                                K                  11                                B                            )                                      1              /              2                                      ,                            Equation        ⁢                                  ⁢        5            
Here the expectation in simple organic smectic A liquid crystals is that the characteristic length will be of the order of the layer spacing until you approach the nematic transition where B (the elastic modulus for compression) rises. (Ref.: de Gennes, P. G., ref 9).).
We would note that the voltages predicted by these equations are threshold voltages, for the first onset of the electro-optic effect. This will be very different (and much lower) than the practical drive voltages required to drive the electro-optic effects at full contrast.
WO 2009/111919 (Halation) discloses an electrically controlled medium for modulating fight includes two plastic thin film layers and a mixture layer is provided between the two thin film layers. The mixture layer consists of smectic liquid crystals, polymeric molecule materials and additives. The liquid crystals used have a polysiloxane chain having a mesogen at one or both of its ends. The polymeric materials appear to be polymerised in situ to divide the space between the film layers into small cells. Conductive electrode layers 4 are provided on the sides of the two plastic thin film layers and the liquid crystal molecules exhibit different alignment states by controlling the size, frequency and acting time of the voltage applied to the conductive electrode layers, so that the electrically controlled medium for modulating light may be switched between a blurredly shielding state and a fully transparent state and even may be switched among a plurality of gradual states of different gray levels. The composition differs from that of the present invention in that it does not include components (c) and (d) of the composition of the present invention and so lacks the properties of the present invention, as discussed below.
EP 0 529 597 (Sumitomo) discloses a liquid crystal display device having a pair of electrodes at least one of which is transparent and a self-supporting liquid crystal film which is placed between the pair of electrodes. The liquid crystal film contains (i) a copolysiloxane backbone where some siloxane units contain a side chain that includes a mesogen (A) and other siloxane units are dimethylsiloxane units that do not include a mesogen (B), (ii) a low molecular weight liquid crystal and (iii) an electrolyte or dopant, which is a tetra-C1-6 alkyl ammonium salt having a bromine counterion. When switched between the clear and opaque states, both the backbone and the mesogenic side chains move and this limits the response speeds. The liquid crystal composition has a smectic A structure but, because the mesogen units are located as side chains along the length of the polysiloxane chain, it lacks the nanoscale siloxane rich sub-layer structure i.e. it does not contain the multi-layered template of the present invention, which is derived in the present invention from oligosiloxane chains having a terminal mesogen group; as more fully discussed below, which gives the present invention its unique properties, which are not shared by the composition of EP 0529597. The topology of the connectivity between the dimethylsiloxane units and the mesogenic units differs significantly between the present invention and EP0529597. In particular, the composition of EP 0529597 will have a poor lifetime and a limited range of operational temperatures, and poor texture and scattering efficiency due to the uncontrolled dilution via the introduction of random dimethylsiloxane units. Furthermore, the compositions are believed to have slow switching speeds; the specification discloses switching speeds but docs not disclose the field applied in order to achieve these speeds and does not disclose the degree of scattering achieved by the switching operations.
U.S. Pat. No. 5,547,604 (Coles) discloses a polysiloxane liquid-crystal having an oligosiloxane chain with a mesogenic terminal grouping. Compositions are described that have a smectic A structure. However, this patent does not disclose the incorporation of a side chain liquid crystal polymer to enhance scattering, as required by the present invention.